Method of forming anisotropic heat spreading apparatus for semiconductor devices

ABSTRACT

A method for forming a heat spreading apparatus for semiconductor devices is disclosed. The method includes extruding a frame material to form multiple individual cells including fillable openings within the frame material; filling a first group of the individual cells with one or more high thermal conductivity materials; filling at least a second group of the individual cells with one or more materials of lower thermal conductivity than in the first group of the individual cells; and implementing a reflowing process following filling the multiple individual cells so as to infiltrate the materials within the individual cells, wherein defined walls of the frame material remain following the reflowing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/160,665 filed Jul. 5, 2005, the contents of which are incorporatedherein in their entirety.

BACKGROUND

The present invention relates generally to integrated circuit heatdissipation devices, and, more particularly, to an anisotropic heatspreading apparatus and method for semiconductor devices.

The semiconductor industry has seen tremendous technological advances inrecent years that have permitted dramatic increases in circuit densityand complexity, as well as equally dramatic decreases in powerconsumption and package sizes. Present semiconductor technology nowpermits single-chip microprocessors with many millions of transistors,operating at speeds of tens (or even hundreds) of MIPS (millions ofinstructions per second), to be packaged in relatively small, air-cooledsemiconductor device packages. Because integrated circuit devices,microprocessors and other related components are designed with increasedcapabilities and increased speed, additional heat is generated fromthese components.

As packaged units and integrated circuit die sizes shrink, the amount ofheat energy given off by a component for a given unit of surface area isalso on the rise. The majority of the heat generated by a component,such as a microprocessor for example, must be removed from the componentin order to keep the component at an acceptable or target operatingtemperature. If the heat generated is not removed from the component,the heat produced can drive the temperature of the component to levelsthat result in early failure of the component. In some instances, thefull capability of certain components cannot be realized since the heatthe component generates at the full capability would result in failureof the component.

An integrated circuit has a front side and a backside. The front side ofthe integrated circuit includes leads for inputs, outputs and power tothe integrated circuit. Leads include many forms, including pins andballs in a ball grid array. The leads of an integrated circuit areattached to pads on another device such as a printed circuit board. Forexample, an integrated circuit that includes a die having amicroprocessor therein has a front side that is attached to the pads ona motherboard, substrate or leadframe. In contrast, a heat sink isattached to the backside of the integrated circuit, extending away fromthe printed circuit board to which the integrated circuit is mounted.Accordingly, a major portion of the heat generated is generallyextracted from the backside of the integrated circuit with the dietherein.

There is a practical limitation on the amount of heat that can beextracted from the backside of the integrated circuit die, as a resultof the thermal resistance of the thermal interface materials (such asany thermal grease, adhesives or solders) used between the backside ofthe integrated circuit die and the heat sink. Typically, heat sinks areformed from materials such as copper or aluminum and have a limitedability to conduct heat. Relatively large fin structures are alsoprovided to increase the amount of heat removed via conduction. Fans arealso provided to move air over the fin structures to aid in the removalof heat. Increasing the size of the fin structure increases the volumeof the heat sink, and generally also increases the stack height of theheat sink. In many electronic devices, the overall size of the heat sinkis generally limited by volume constraints of the housing. For example,in some mobile products such as laptop computers and ultra-mobilecomputers, small stack heights are required.

The use of aluminum and copper heat sinks with fin structures are nowtherefore approaching their practical limits for removal of heat from ahigh performance integrated circuit, such as the integrated circuitsthat include dies for microprocessors. When heat is not effectivelydissipated, the dies develop “hot spots” (i.e., areas of localizedoverheating). Unfortunately, the current cost performance lids do notadequately solve this heat dissipation/distribution problem. Moreover,the existing lid materials are isotropic, in that they provide singularheat flow characteristics derived from the intrinsic homogeneousproperties of the lid material. In some instances, traditional aluminumand copper heat sinks have been replaced with sinks incorporating exoticmaterials (e.g., diamond particles) therein. However, diamond heat sinksare difficult to manufacture, in addition to being expensive. Inparticular, one aspect of diamond heat sink formation is that one majorsurface of the heat sink must be ground smooth in order to provide agood thermal connection at a thermal interface. The grinding orsmoothing of diamond is also time consuming.

In view of the above, it would be desirable to be able to provide a heatspreading apparatus and methodology for semiconductor devices in amanner that is both efficient and cost effective.

SUMMARY

The foregoing discussed drawbacks and deficiencies of the prior art areovercome or alleviated by a heat spreading apparatus for use in coolingof semiconductor devices. In an exemplary embodiment, the heat spreadingapparatus includes a frame having a plurality of individual cells formedtherein, each of the cells configured for filling with a material ofselected thermal conductivity therein. The selected thermal conductivityof material within a given one of the cells corresponds to a thermalprofile of the semiconductor device to be cooled.

In another embodiment, a method for forming a heat spreading apparatusfor semiconductor includes extruding a frame material to form aplurality of individual cells, filling a first group of the individualcells with one or more high thermal conductivity materials, and fillingat least a second plurality of the individual cells with one or morematerials of lower thermal conductivity than in the first plurality ofcells.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1 is a top, cross sectional view of the structure of a heatspreading apparatus, in accordance with an embodiment of the invention;

FIG. 2 illustrates one specific example of the heat spreading apparatusof FIG. 1, following the fill of the individual cells with variousthermal spreading materials;

FIG. 3 illustrates an alternative embodiment of the fill pattern of theheat spreading apparatus;

FIG. 4 illustrates another alternative embodiment of the fill pattern ofthe heat spreading apparatus;

FIG. 5 illustrates still another alternative embodiment of the fillpattern of the heat spreading apparatus;

FIGS. 6-8 illustrate partial, side cross sectional views of variousadditional embodiments of the heat spreading apparatus;

FIG. 9 is a flow diagram illustrating a method for forming a heatspreading apparatus, in accordance with a further embodiment of theinvention; and

FIG. 10 is a flow diagram illustrating an alternative embodiment of themethod shown in FIG. 9.

DETAILED DESCRIPTION

Disclosed herein is a cost/performance heat spreading apparatus andmethod that provides tailored thermal conduction properties of selectiveareas of the apparatus to correspond to determined hot spots on a chip.Briefly stated, the heat spreading apparatus incorporates anisotropicproperties to address the varying heat transfer requirements of asemiconductor chip, while a web (e.g., honeycomb) of high strengthmaterial included in the heat spreading apparatus provides the skeletonthereof with sufficient strength to be used in mechanically loadedapplications, such as land grid array (LGA) applications.

Moreover, the present solution allows high thermal conductivity (T_(c))materials, such as diamond particles in a polymer matrix, tospecifically contact the hot spots of a chip, while higher strength,less thermally conductive materials may be used to fill the honeycombcells where the desired heat transfer is less significant. Byspecifically tailoring the material that each of the honeycomb cells arefilled with, more expensive, high heat transfer materials can beprovided only where desired in specific locations, thus saving the costof fabricating a lid entirely with more expensive materials (such asdiamond, for example).

Referring now to FIG. 1, there is shown a top, cross sectional view ofthe structure of a heat spreading apparatus 100, in accordance with anembodiment of the invention. As is shown, the heat spreading apparatus100 includes a web of individual cells 102 that may be filled withmaterials of various thermal properties. In the embodiment depicted, theweb is shown a “honeycomb-like” design of individual hexagonal cells.However, other shapes and designs of cell configurations are alsocontemplated including, but not limited to, packed arrangements such ascircular type cell shapes (e.g., circle, oval, elliptical, etc.) andpolygonal type cell shapes having three or more sides, as well as cellsof varying sizes. Exemplary materials that may be used to form thesidewall, top and bottom surfaces of the cells 102 include, but are notlimited to, metals (e.g., aluminum, copper), thermoplastics and platedmaterials. The web can further be extruded, molded, machined and thelike, depending on the particular material or materials used in formingthe same. It should be further noted that in the exemplary embodiment ofFIG. 1, the thickness of the cell walls are not necessarily to scale,and are exaggerated for illustrative purposes.

FIG. 2 illustrates one specific example of the heat spreading apparatus100 of FIG. 1, following the fill of the individual cells 102 withvarious thermal spreading materials used therein. In the exampledepicted, a first group of cells 102 has a first fill material 104 aformed therein, a second group of cells 102 has a second fill material104 b formed therein, a third group of cells 102 has a third fillmaterial 104 c formed therein, and a fourth group of cells 102 has afourth fill material 104 d formed therein. Further, in the exampledepicted, the chip (not shown) to which the heat spreading apparatus 100is to be attached is determined to have the hottest spots in the centerthereof, with the cooling requirements being decreased in a generallyrectangular pattern away from the center. Accordingly, for an economicalyet effective heat transfer apparatus 100, the first cell material 104 a(darkest shading) may be a high T_(c), higher cost material such asdiamond, silicon carbide or aluminum silicon carbide, for example. Forthe cells corresponding to somewhat cooler spots on the chip, the secondand third cell materials 104 b, 104 c (intermediate shadings) may bemedium cost, medium T_(c) materials. Finally, for the peripheral areasof apparatus 100 corresponding to the coolest spots on the chip, thefourth cell material 104 d (lightest shading) may be a low cost, lowT_(c) material.

FIG. 3 illustrates an alternative embodiment of the heat spreadingapparatus 100, wherein the pattern of cell fill materials varies in acircularly outward pattern, again with the higher T_(c)/cost fillmaterial 104 a being located in the centermost cells. In FIG. 4, a“sunburst” pattern of high T_(c) fill material 104 a is defined withinthe heat spreading apparatus 100. Again, several different patterns ofdiffering fill materials are contemplated, depending on a specificthermal profile and mechanical needs of the chip to which the apparatus100 is intended to be attached. FIG. 5 illustrates still an alternativeembodiment of the heat spreading apparatus 100, having cell patterns ofboth different fill materials and different shapes. For example, theheat spreading apparatus 100 can have cells 102 of a first size andshape (such as small hexagonal or circular cells), as well as cells 106of a second size and shape (such as larger rectangular cells). Thusconfigured, the heat spreading apparatus 100 can optimize thermalspreading, strength of the structure (for retention hardware forceswhere applicable), and heat dissipation of specific chip hot spots.

As indicated above, the individual cells 102 of the heat spreadingapparatus 100 may be filled with a variety of different materials tocreate anisotropic thermal conduction properties across the entirestructure in the z (as well as the planar x-y) directions. To this end,thermoplastics and thermosets may be compounded with industrial diamondparticles, graphite, liquid metals, water or phase change materials. Inessence, the cells 102 may be filled (either fully or partially) withany material that provides the desired thermal and mechanical propertiesin the chip location specified.

FIGS. 6-8 illustrate partial, side cross sectional views of variousadditional embodiments of the heat spreading apparatus 100. Forinstance, in FIG. 6, it can be seen that the higher T_(c) fill materials104 a are used in the center cells, while the intermediate and lowerT_(c) fill materials 104 c, 104 d are used in the middle and outerperimeter cells, respectively. As is further illustrated, the heatspreading apparatus 100 may include a top surface 108 and a bottomsurface 110 for providing an enhanced thermal interface to a heat sinkand chip, as well as to encapsulate the filler material(s) used withinthe cells. FIG. 7 once again illustrates that the cells may havedifferent shapes and/or sizes with respect to one another. As furthershown in FIG. 8, one or more of the cells may be partially filled with athermal liquid (e.g., water) so as to effectively act as a vapor chamberand/or “heat pipe.”

FIGS. 9 and 10 illustrate exemplary heat spreader formation processes,in accordance with a further embodiment of the invention. In theembodiment of FIGS. 9( a) and 9(b), the exemplary process 900 isillustrated for a simple 5-cell spreader structure having a center cellbordered by the other four cells. However, it will be appreciated thatthis example is simplified for illustrative purposes only, and that theprocess is applicable to a heat spreading device having a greater numberof individual cells, such as those described above. In this particularembodiment, the heat spreading device is created by infiltrating a moldfilled with thermally conductive particles, with different regions ofthe spreader having different fill materials, wherein the fill materialsare initially separated by a frame. The frame (or cell walls) is“consumable” in this embodiment, meaning that they are absorbed into theother spreader materials during fabrication. FIG. 9( a) is a processflow diagram, while FIG. 9( b) illustrates the simplified cell structureof the heat spreading apparatus.

As particularly shown in block 902 of FIG. 9( a), the frame is extruded(e.g., using a material such as silver) and placed into a mold forinfiltration of the individual cells. The center cell (or cells) isfilled with a high T_(c) material, such as high-grade diamond particles,as shown in block 904. In contrast, the border cells are filled with alower T_(c) material, such as silicon carbide, as shown in block 906.Then, the filled mold and source of bulk frame material (e.g., silver)is placed within a suitable heating apparatus, as shown in block 908. Inblock 910, the assembly is reflowed to as to infiltrate the particleswithin the mold. Additionally, with this particular embodiment, theframe walls are melted so as to form an amalgam with the fill materials.However, the tailored thermal profile of the apparatus is stillpreserved. Next, the assembly is cooled and removed from the mold, asshown in block 912, and any flash may be removed from the assembly bymachining and/or surface finishing, as shown in block 914.

Finally, FIG. 10( a) illustrates an alternative process flow diagram1000 similar to that of FIG. 9( a), only with the cell wall structurebeing maintained following cell material infiltration. In block 1002,the frame is extruded (e.g., using a material such as copper). The framemay also be chrome plated before being placed into a mold forinfiltration of the individual cells. The center cell (or cells) isfilled with a high T_(c) material, such as high-grade diamond particles,as shown in block 1004. In contrast, the border cells are filled with alower T_(c) material, such as low-grade diamond particles, as shown inblock 1006. Then, the filled mold and source of bulk frame material(e.g., copper) is placed within a suitable heating apparatus, as shownin block 1008. In block 1010, the assembly is reflowed to as toinfiltrate the particles within the mold. Again, in this particularembodiment, the frame walls are not melted during the reflow. Next, theassembly is cooled and removed from the mold, as shown in block 1012,and any flash may be removed from the assembly by machining and/orsurface finishing, as shown in block 1014.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A method for forming a heat spreading apparatus for semiconductordevices, the method comprising: extruding a frame material to form aplurality of individual cells comprising fillable openings within theframe material; filling a first group of said individual cells with oneor more high thermal conductivity materials; filling at least a secondgroup of said individual cells with one or more materials of lowerthermal conductivity than in said first group of said individual cells;and implementing a reflowing process following filling said plurality ofindividual cells so as to infiltrate said materials within saidindividual cells, wherein defined walls of said frame material remainfollowing said reflowing.
 2. The method of claim 1, wherein saidplurality of individual cells are formed in at least one of ahoneycomb-like arrangement and a packed arrangement.
 3. The method ofclaim 2, wherein said individual cells are formed in at least one of acircular type shape and a polygonal type shape having three or moresides.
 4. The method of claim 1, wherein said individual cells areconfigured with varying sizes according to said thermal profile.
 5. Themethod of claim 1, wherein said frame material comprises one of more of:a metal material and a thermoplastic material.